Final answer:
The lac repressor binds to the operator site in the lac operon, preventing gene transcription. However, when lactose is present, it binds to the lac repressor, causing it to dissociate from the operator site and allowing gene transcription to occur.
Step-by-step explanation:
The lac repressor (under the right conditions) binds to the operator site in the lac operon. The operator site is a region of DNA that acts as a switch for gene transcription. When the lac repressor binds to the operator site, it prevents RNA polymerase from transcribing the structural genes of the lac operon.
However, when lactose is present, allolactose (a product of lactose metabolism) binds to the lac repressor, causing it to dissociate from the operator site. This allows RNA polymerase to bind to the promoter and initiate transcription of the structural genes. The lac operon is a classic example of an inducible operon, where the presence of lactose induces gene transcription.